1. Field of the Invention
The present invention relates to a method for manufacturing a laser device that is used for a light source of an optical disk drive, a tester, an illuminator, an analyzer or the like. In particular, the present invention relates to a method for manufacturing a shortwave laser device such as a gallium nitride semiconductor laser device that emits a laser beam of blue color, violet color, near-ultraviolet or ultraviolet in a short wavelength range.
2. Description of Related Art
A nitride semiconductor in the group III such as GaN, AlGaN, GaInN and AlGaInN (hereinafter referred to as a “gallium nitride system semiconductor”) has a larger energy band gap than an AlGaInAs system semiconductor or an AlGaInP system semiconductor. Therefore, such semiconductor materials can emit light having a short wavelength and are superior in light emission efficiency since they are a direct transition-type. The gallium nitride system semiconductor having these characteristics has been receiving attention as a material that constitutes a semiconductor light emission element such as a semiconductor laser device that is capable of emitting light over a wide wavelength range from ultraviolet to green color or a light emission diode (LED) that can cover a wide light emission wavelength range from ultraviolet to red color. Application areas thereof cover a wide range that includes a high density optical disk drive, a full color display, an environment field and a medical field.
The gallium nitride system semiconductor has a higher thermal conductivity than a GaAs system semiconductor or the like, so it is expected to be used as an element that works for high temperature and high output operation. In addition, while an AlGaAs system semiconductor contains harmful arsenic (As) and a ZnCdSSe system semiconductor contains harmful cadmium (Cd), the gallium nitride system semiconductor does not contain such a harmful material. Furthermore, ammonia (NH3) that is used for manufacturing a gallium nitride system semiconductor has lower toxicity than arsine (AsH3) that is used for manufacturing an AlGaAs system semiconductor, so it is a material that causes little stress on the environment.
A shortwave semiconductor laser device such as a gallium nitride system semiconductor laser device is generally manufactured by sealing a semiconductor laser chip hermetically with a cap that is transparent for a laser beam. In this manufacturing process, a contaminant may intrude into the inside space of the semiconductor laser chip and the cap, and the contaminant may be deposited on a light emitting face of the semiconductor laser chip so that laser characteristics are deteriorated. This is a problem called a degradation of a light emitting end face.
The above-mentioned contaminant is regarded to be a siloxane system material, a hydrocarbon compound or the like. A source of the contaminant is regarded to include a human body, a microorganism, a construction material, grease or oil that is used for a manufacturing machine and atmosphere of the manufacturing process. Furthermore, a pressure sensitive adhesive sheet that is used in the manufacturing process of the laser device can be a source of the contaminant. Once a contaminant adheres to the laser chip or the cap during the production of the laser device, the contaminant inside the cap cannot be removed even if the step of sealing the laser chip inside the cap is performed in a clean room.
For example, it is considered that a contaminant contained in an adhesive agent that is used for the pressure sensitive adhesive sheet is deposited on the light emitting end face in the following steps. In the manufacturing process of the gallium nitride system semiconductor laser chip, a wafer on which a plurality of gallium nitride system semiconductor laser chips are formed is divided into bars having the light emitting end face (hereinafter each of the bars is referred to as a “laser bar”). This laser bar is further divided into individual gallium nitride system semiconductor laser chips in a chip dividing step. In this step, the laser bar is temporarily adhered to the pressure sensitive adhesive sheet. Next, scribing lines are formed on the laser bar by using a diamond scribing tool or the like. Then, the laser bar is divided into chips. In this case, since the laser bar is adhered to the pressure sensitive adhesive sheet, the divided chips of the gallium nitride system semiconductor laser are prevented from falling to pieces.
The divided gallium nitride system semiconductor laser chips are separated from the pressure sensitive adhesive sheet one by one chip. On this occasion, the adhesive agent of the pressure sensitive adhesive sheet may remain on the gallium nitride system semiconductor laser chip. The individual gallium nitride system semiconductor laser chip separated from the pressure sensitive adhesive sheet is fixed to a stem after a chip test is performed. Further, a cap is attached so that the gallium nitride system semiconductor laser chip is sealed hermetically inside a package to be a completed laser device. It is considered that before the hermetical sealing, organic substances may adhere to other elements such as a sub mount, the stem and the cap.
Inside the package of the completed laser device, the adhered contaminant may be vaporized and floating. Then, the vaporized contaminant may cause a photochemical reaction by a photo chemical vapor deposition (CVD) effect due to a shortwave laser beam emitted from the gallium nitride system semiconductor laser chip, so as to generate a photochemical reaction substance containing silicon (Si) or carbon (C) as a main component. This photochemical reaction substance is deposited on the light emitting end face in which light intensity is the highest. In addition, heat due to operation of the laser device causes convection of filler gas, so that photochemical reaction substances contained in the filler gas can be supplied continuously to the light emitting end face of the laser chip.
This phenomenon becomes conspicuous in a laser having an oscillation wavelength (a light emission wavelength) at 550 nm or shorter that has high energy per photon and facilitates to promote chemical reaction, particularly in a laser device that uses a gallium nitride system semiconductor laser chip having an oscillation wavelength at 420 nm or shorter.
When a photochemical reaction substance is deposited on the light emitting end face as described above, it causes increase of light absorption and fluctuation of a reflection factor on the light emitting end face. As a result, drive current of the laser that is necessary for obtaining the same power may increase, and a life of the laser may be shortened substantially.
In order to avoid this problem, there is a technique disclosed in JP-A-2004-14820, which removes a contaminant floating in a filler gas so as to prevent deposition of a reaction substance on the light emitting end face. This technique uses a zeolite adsorbent that is disposed inside the semiconductor laser device that is sealed with a cap. However, according to a result of an experiment performed by the inventor, the contaminant in the filler gas could not removed sufficiently by using this technique. It was found that a reaction substance was deposited on the light emitting end face of the semiconductor laser chip during a long period operation.
As another prior art, JP-A-2004-40051 discloses a method of sealing a stem to which the gallium nitride system semiconductor laser chip is fixed, hermetically with a cap, after removing a contaminant adhered to the gallium nitride system semiconductor laser chip and the cap, by irradiating a ultraviolet ray to the stem, or by irradiating a plasma generated by an electron cyclotron resonance (ECR) method was used. However, according to a result of an experiment performed by the inventor, it was found that the semiconductor laser chip itself was damaged by the energy beam so that degradation such as increase of operating voltage occurred when using this method. In addition, if a component such as the stem or the gallium nitride system semiconductor laser chip is exposed to air after irradiating an ultraviolet ray or plasma, the component may be contaminated again by the contaminant before sealed hermetically. In particular, the re-contamination becomes conspicuous if there is a step that uses an Ag paste containing much organic solvent between a step of irradiating an ultraviolet ray or a plasma and a step of hermetical sealing, or if there is a long period of time for exposing the component to air between the step of irradiating an ultraviolet ray or a plasma and the step of hermetical sealing.
As still another prior art, JP-A-2004-273908 discloses a method of removing a contaminant that is adhered to a gallium nitride system semiconductor laser chip and a cap by sealing a stem to which the gallium nitride system semiconductor laser chip is fixed, hermetically with the cap in an ozone atmosphere, and then irradiating a ultraviolet ray. However, according to a result of an experiment performed by the inventor, it was found that a metal that was used for the semiconductor laser chip except for Au and Pt was oxidized so that degradation such as increase of operating voltage occurred when this method was used. For example, if a solder that fixes the gallium nitride system semiconductor laser chip to the stem or an electrode that is used for the gallium nitride system semiconductor laser chip is oxidized by the ozone, degradation of characteristics or shortening of life may occur. On the other hand, if only the irradiation of the ultraviolet ray is performed simply without the hermetical sealing in the ozone atmosphere, sufficient effect cannot be obtained. It was found that the degradation (increase of drive current) would occur when a continuous operation was performed at an ambient temperature of 60° C. in particular.